This invention relates generally to gas turbine engine variable geometry exhaust nozzles and, more particularly, to seal systems used to shield the gas turbine engine variable geometry systems.
At least some known gas turbine engines include an exhaust nozzle including a variable geometry system. The variable geometry system adjusts an area of the exhaust nozzle. Because the exhaust nozzles are subjected to high temperatures and thermal gradients as a result of hot combustion gases exiting the engine, the variable geometry systems include seal systems to shield components of the variable geometry system. Additionally, the seal systems form a portion of an exhaust flowpath through the engine.
At least some known seal systems include a backbone and a base sheet. The backbone secures the base sheet within the variable geometry system. To facilitate extending a useful life at high temperature operation, at least some known base sheets are fabricated from non-metallic materials, such as ceramic matrix composite (CMC) materials.
At least some known CMC base sheets are attached to the backbone using mechanical fasteners, such as rivets or bolts. Over time, continued thermal expansion may create local stress concentrations within the bolted and riveted joints. Furthermore, continued thermal cycling may loosen such mechanical fasteners.
Other known CMC base sheets are attached to the backbone with embedded attachments. Such attachments rely primarily on an interlaminar tensile strength of the CMC material to remain in place. Because such tensile strength may be a weakest load path through the base sheet, continued thermal cycling may loosen such embedded attachments and induce potentially damaging stresses into the base sheet.
In an exemplary embodiment, a gas turbine engine exhaust nozzle includes a variable geometry system including a seal system that facilitates extending a useful life of the engine variable geometry system. The seal system includes a backbone and an attachment assembly. The attachment assembly frictionally engages the backbone to secure the attachment assembly to the backbone. The attachment assembly includes a plurality of clamp sub-assemblies and a non-metallic base sheet. The clamp sub-assemblies are clamped through openings extending through the base sheet, such that a predetermined amount of preload force is applied to the base sheet. In the exemplary embodiment, the base sheet is fabricated from a ceramic matrix composite material.
During operation, as high temperature combustion gases pass through the exhaust nozzle, the seal system shields components of the variable geometry system from the high temperatures, and the clamp sub-assemblies transfer downward seal loading to the backbone. Over time, as the seal system is exposed to high temperatures, the clamp sub-assemblies permit the attachment assembly to thermally expand axially without breaching sealing between the base sheet and the backbone. Furthermore, because the clamp sub-assemblies provide a margin for thermal and mechanical misalignments, the attachment assembly facilitates eliminating local stress risers in the seal system, thus facilitating extending the useful life of the seal system.